Gaseous waste products resulting from the combustion of hydrocarbonaceous fuels, such as gasoline and fuel oils, comprise carbon monoxide, hydrocarbons and nitrogen oxides as products of combustion or incomplete combustion, and pose a serious health problem with respect to pollution of the atmosphere. While exhaust gases from other carbonaceous fuel-burning sources, such as stationary engines, industrial furnaces, etc., contribute substantially to air pollution, the exhaust gases from automotive engines are a principal source of pollution. Because of these health problem concerns, the Environmental Protection Agency (EPA) has promulgated strict controls on the amounts of carbon monoxide, hydrocarbons and nitrogen oxides which automobiles can emit. The implementation of these controls has resulted in the use of catalytic converters to reduce the amount of pollutants emitted from automobiles.
In order to achieve the simultaneous conversion of carbon monoxide, hydrocarbon and nitrogen oxide pollutants to innocuous gases, it has become the practice to employ exhaust gas catalysts in conjunction with air-to-fuel ratio control means which functions in response to a feedback signal from an oxygen sensor in the engine exhaust system. This type of catalyst is commonly called a three component control catalyst. The air-to-fuel ratio control means is typically programmed to provide fuel and air to the engine at a ratio at or near the stoichiometric balance of oxidants and reductants in the hot exhaust gases at engine cruising conditions, and to a stoichiometric excess of reductants during engine startup and at engine acceleration conditions. The result is that the composition of the exhaust gases with which the catalyst is contacted fluctuates almost constantly, such that conditions to which the catalyst is exposed are alternatively net-reducing (fuel rich) and net-oxidizing (fuel lean). A catalyst for the oxidation of carbon monoxide and hydrocarbons and the reduction of nitric oxide must be capable of operating in such a dynamic environment.
The exhaust gas also contains other components such as sulfur oxides, phosphorus and zinc compounds which are known catalyst poisons. The sulfur oxides present in the exhaust stream can react with the catalyst to form other products. For example under fuel lean (net-oxidizing) conditions, sulfur dioxide (SO.sub.2) reacts with oxygen (O.sub.2) over the catalyst to form sulfur trioxide (SO.sub.3) which is then converted to sulfates (SO.sub.4.sup.=) by reaction with water. Under fuel rich (net-reducing) conditions the SO.sub.2 reacts with hydrogen (H.sub.2) to form hydrogen sulfide (H.sub.2 S). The formation of H.sub.2 S is particularly objectionable because of its strong odor.
In addition to the formation of H.sub.2 S over noble metal catalyst, a storage phenomenon has also been observed. This storage phenomenon has been documented in the literature, G. J. Barnes and J. C. Summers, "Hydrogen Sulfide Formation Over Automotive Oxidation Catalysts", Society of Automotive Engineers, Paper No. 750093. The experimenters showed that sulfur accumulated on noble metal catalysts under both oxidizing and reducing atmospheres. For example, under oxidizing conditions the sulfur is typically stored as sulfates (SO.sub.4.sup.=) which is converted to H.sub.2 S under reducing conditions.
Although this phenomenon has been recognized for many years, the problem which it generates, i.e. unpleasant odor, was relatively minor and was not of much concern until recently. In the past few years automotive catalyst technology has improved so that the catalysts are much more active than previous catalysts. Part of this improvement has been achieved by increasing the content of the rare earths (hereinafter referred to as a first active component) present in the catalytic composite. Unfortunately, the rare earths appear to increase the storage of sulfur during fuel lean operation, and when release occurs the concentration of hydrogen sulfide is much larger than would have been anticipated, based on the sulfur content of the fuel. Consequently, the resultant odor is quite noticeable and many more drivers are offended by the increased hydrogen sulfide odors.
Since the odor has become more noticeable and objectionable, a need exists to minimize the hydrogen sulfide emissions from catalyst equipped automobiles. The instant invention cures this problem by providing a catalytic composite which contains a metal that forms stable metal sulfides (hereinafter referred to as a second active component) under rich conditions and therefore minimizes the release of hydrogen sulfide under rich conditions. Some of the metals which are very effective in this regard are lead, zinc, copper, nickel, and cobalt.
Catalysts which contain second active components such as nickel are well known in the art. For example, U.S. Pat. Nos. 3,840,471 and 3,903,020 teach the use of precious metals in combination with rare earth stabilizers and promoters such as nickel. However, the prior art disclosed the use of these second active components as promoters. That is, these metals are capable of promoting reactions such as the water gas shift and steam reforming reactions. No mention has been made in the art about using such second active components to minimize the amount of hydrogen sulfide which is formed over conventional automotive catalysts. Therefore, the present invention presents a new use of these second active components and fills a need in the field of automotive catalysts.
Additionally, the usual manner of depositing said second active components is to uniformly distribute these components on a high surface area refractory inorganic oxide support which is usually alumina and which also has noble metals deposited thereon. However, when these catalysts are exposed to high temperatures (&gt;750.degree. C.), the second active components can interact with the alumina support to form metal aluminates. These aluminates are very stable and are not active for the desired reactions. This means that the catalytic composite will lose activity upon exposure to such high temperatures, due to changes in the second active components. Another factor is that while components such as nickel, cobalt, etc. promote some reactions in conjunction with the noble metal species, they also act as a poison to the noble metal components for other reactions. Both of these phenomena are well known in the art.
The instant invention also cures this problem by depositing the second active component on a secondary refractory inorganic oxide such as zirconia, ceria, titania, etc. Since the second active components are metals, this means that metal aluminates are not formed at high temperatures because the metals do not interact with either zirconia, ceria or titania, and are not in direct contact with the alumina support. Secondly, because the second active components are not in intimate contact with a majority of the noble metal components, there is less detrimental interaction between the second active component and the noble metal species. An improved catalytic composite is thereby produced by the instant invention.
In certain applications nitric oxides are not a problem and therefore a catalyst need only oxidize the hydrocarbons and carbon monoxide to carbon dioxide and water. This type of catalyst is commonly called an oxidation catalyst. Additionally, some applications may require both a three component control catalyst and an oxidation catalyst. In both of these possible applications, the exhaust gas may become fuel-rich and thus present the same problem as for a three component control catalyst. The instant invention also cures the H.sub.2 S problem associated with an oxidation catalyst.